The efficient production of oil and gas from subsea wells requires the injection of various treatment chemicals to maintain the desired composition of well fluid by controlling variables such as corrosion, scale, paraffin, emulsion, and hydrates. Often, several wells are located near each other within a producing field, but at significant distance from a surface pumping station from which chemicals are pumped. In many instances wells are offset from the pumping station by more than 10 miles, and at depths of more than 900 feet. Reliable methods and systems are therefore required to distribute chemicals to each well.
Existing chemical injection control systems are typically based on a pressure compensated flow device using a pressure regulating valve in combination with an orifice to regulate the chemical flow at each well. Flow through a capillary orifice is often adjusted using a tapered metering screw to adjust an orifice diameter. A major disadvantage of this type of system stems from the small orifice size required. Chemicals are typically needed only in small quantities, but they must be delivered at high pressure to ensure flow to every well over long distances. To deliver a chemical at several thousand psi at the rate of only a few gallons per day requires a very small orifice. An orifice this small is easily clogged by contaminants. Some prior art includes flow filters to prevent clogging, but providing and servicing these filters, especially in subsea environments, is expensive.
U.S. Pat. No. 4,512,187 discloses an example of a chemical injection system. Two displacement chambers are provided and connected by a control conduit. Damping fluid is contained between first and second movable barriers in the first and second chambers respectively. The damping fluid can pass between the first and second chambers via the control conduit. A pressure and control valve is included in the control conduit for controlling flow of the damping fluid. Chemical fluid to be delivered to a well enters one end of the first displacement chamber opposite the first movable barrier from the damping fluid. This moves the first movable barrier to displace the damping fluid to the second chamber, which in turn moves the second movable barrier to dispense the chemical fluid opposite the second movable barrier from the damping fluid. The flow of chemical fluid to and from the chambers is selectively reversed, to provide continuous flow of chemical fluid.
U.S. Pat. No. 4,512,188 discloses another example of a chemical injection system intended to reduce shear forces on the chemical fluid to be delivered. Each of first and second piston and cylinder assemblies has a first port on one side of the piston and a second port on the other side. A secondary fluid path between the two second ports contains a damping fluid directed through a pressure reducing valve. The rate of flow of the primary fluid from the discharge cylinder is controlled by the rate of flow of the damping fluid through the pressure reducing valve. A four way valve couples the chemical fluid at relatively high pressure through the first port in a first cylinder, and the controlled liquid is discharged at relatively low pressure from the first port in the second cylinder.
The systems disclosed in the '187 and '188 patents are similar in that they are designed specifically for shear-sensitive fluids and thus require passing a separate damping fluid through a control valve. An associated disadvantage of this type of system is thus the need for a separate damping fluid, along with increased parts, such as two separate cylinders each housing separate fluid barriers. A system with two cylinders and fluid barriers is inherently more prone to failure than a system with fewer parts. Another disadvantage is the risk of mixing the damping fluid with and contaminating the chemical fluid to be delivered.
Another complication of existing systems in general is that using a small orifice increases the need to verify flow rate data provided by flow control devices. A separate feedback device is commonly used at the well for this purpose. These devices operate over a narrow range and are therefore limited in application. This further increases the cost of chemical injection systems.